In a shared medium optical network, such as a passive optical network (PON), which comprises multiple optical sources transmitting toward a single optical receiver, some method must be used to prevent data transmitted by one transmitter from interfering with data transmitted by the other transmitters. In this type of multipoint-to-single point network there are two methods commonly used to solve this problem: time division multiple access (TDMA) and wavelength division multiplexing (WDM). TDMA is used when all of the transmitters share a common optical wavelength or wavelength band. WDM is used when each transmitter uses a unique optical wavelength or wavelength band which does not interfere with those used by the other transmitters.
In TDMA optical systems each optical source transmits by bursting its information onto the common physical medium. Transmissions from different sources are made possible by offsetting, in time, the burst from each source so that none of the bursts overlaps, in time, with any other burst from any source. Otherwise, the transmissions from two or more sources could collide at some common point in the network, causing loss of data. The time allocated for a single burst of data from a given transmitter is called a xe2x80x9ctimeslotxe2x80x9d. In an optical TDMA network, all the transmitters typically use the same band of wavelengths with no separation among the sources in the optical frequency domain. This approach is referred to as xe2x80x9csingle wavelength TDMAxe2x80x9d.
In current WDM systems, the transmission mode used is a continuous one in which each transmitter is enabled all the time and is continuously modulated by on-off keying. There is no separation among sources in the time domain, but there is enough separation among sources in the optical frequency domain so that the wavelength of a given transmitter does not interfere with the wavelengths of the other transmitters at the optical receiver. This approach is referred to as xe2x80x9ccontinuous mode WDMxe2x80x9d.
To increase network capacity and the number of transmitters that can share a multipoint-to-single point optical network, it would be advantageous to combine the sharing in the time domain provided by TDMA and the sharing in the optical wavelength/frequency domain provided by WDM.
The present invention relates to a method and system for using burst mode transmission on multiple optical wavelengths. This combination is referred to herein as xe2x80x9cburst mode WDMxe2x80x9d or, equivalently, xe2x80x9cmultiple wavelength TDMAxe2x80x9d.
Accordingly, a method of communicating between a central terminal and plural remote terminals over a passive optical network having downstream and upstream portions includes transmitting burst data signals from remote terminals to the central terminal over the upstream network portion. A first group of remote terminals transmits burst data signals in respective first timeslots that are multiplexed at a first optical wavelength. A second group of remote terminals transmits burst data signals in respective second timeslots that are multiplexed at a second optical wavelength.
According to an embodiment, each upstream wavelength carrying burst mode transmissions can be spectrally spaced as close as possible to adjacent wavelengths. Accordingly, a method of communicating between a central terminal and plural remote terminals includes transmitting a synchronization signal from the central terminal to the remote terminals over the downstream network portion and transmitting burst data signals from remote terminals to the central terminal over the upstream network portion. A first group of remote terminals transmits burst data signals in respective first timeslots that are synchronized to the received synchronization signal and multiplexed at a first optical wavelength. A second group of remote terminals transmits burst data signals in respective second timeslots that are synchronized to the received synchronization signal and multiplexed at a second optical wavelength. The first and second timeslots each include an active period during which burst data signals are transmitted and a guard period during which burst data signals are not transmitted. The first and second timeslots are synchronized such that respective active and guard periods are phase aligned with each other.
According to another aspect, a method of communicating between a central terminal and plural remote terminals over an optical network includes transmitting a TDM data signal having a synchronization signal from the central terminal to the remote terminals. Burst data signals are transmitted over the optical network from N groups of the remote terminals to the central terminal in respective TDMA timeslots that are synchronized to the received synchronization signal with each of the N groups operating at a different optical wavelength.
Another aspect of the present synchronization approach includes transmitting a common synchronization signal in each of a plurality of downstream TDM signals at respectively different optical wavelengths. Each remote terminal receives at least one of the plural downstream TDM signals containing the common synchronization signal.
A communication system in accordance with the present invention includes a passive optical network having downstream and upstream portions, a central terminal, and plural remote terminals coupled to the passive optical network for communicating with the central terminal. The central terminal transmits a synchronization signal to the remote terminals over the downstream network portion. A first group of the remote terminals transmits burst data signals to the central terminal in respective first timeslots that are synchronized to the received synchronization signal and multiplexed over the upstream network portion at a first optical wavelength. A second group of the remote terminals transmits burst data signals to the central terminal in respective second timeslots that are synchronized to the received synchronization signal and multiplexed over the upstream network portion at a second optical wavelength.
An advantage of the synchronization approach is that by avoiding interference between signals transmitted at closely spaced adjacent wavelengths, directly modulated laser transmitters can be used rather than more costly externally modulated laser transmitters.